1. Field of the Invention
The present invention relates to semiconductor elements and the manufacture of them and, in particular, to polysilicon bipolar transistors having an improved base terminal architecture, and the manufacture of them.
2. Description of the Related Art
Modern integrated bipolar circuits are characterized by high switching speed, high transit frequencies, good driver features, high transconductance of the transistors and high stability of the control voltage. Due to these features, bipolar technology has a high importance in micro-electronics. With present conventional manufacturing processes of integrated bipolar circuits, a processing of data rates of more than 10 Gbits/sec can, for example, be achieved at present, wherein bipolar transistors having an implanted base with transit frequencies of 30 GHz can be realized using the polysilicon emitter and base technology.
The base terminal structure is, among other things, decisive for good electrical features of polysilicon bipolar transistors. In a base terminal structure of a conventional polysilicon bipolar transistor, as is exemplarily illustrated in FIG. 3, a polysilicon layer 32 which is, after being patterned, effective as the base terminal region is generally applied on a single-crystal silicon substrate 30 in which the collector region is formed. An insulation layer 34, such as, e.g., silicon oxide, is applied over the polysilicon layer 32. A so-called emitter window 38 is usually formed in double polysilicon bipolar transistors having an implanted base region 36, wherein this emitter window 38 is provided to form, on the one hand, the active emitter region of the bipolar transistor in the silicon substrate 30 and, on the other hand, the emitter terminal for this region. The emitter window 38 is thus formed by etching through the insulation layer 34 and subsequently through the polysilicon material 32 serving as the base terminal region. Since the polysilicon material 32 of the base terminal region is directly on top of the single-crystal silicon substrate 30, the etching process cannot be stopped abruptly on the single-crystal silicon substrate 30 due to the lacking etching selectivity between the poly-crystal silicon material 32 and the single-crystal semiconductor material of the silicon substrate 30.
For this reason, a considerable substrate etching in an order of magnitude of about 40 nm is provided in the region of the emitter window 38 due to the resulting layer thickness variations of the polysilicon and insulation layers to be etched and due to etching rate differences over the silicon wafer, to etch, definitely on each silicon wafer and everywhere on the different silicon wafers to be processed, through the poly-crystal base terminal layer. The active base region 36 of the bipolar transistor, which subsequently will be referred to as the “intrinsic” base region, is implanted in the silicon substrate 30 in the region of the emitter window 38. Subsequently, an inner spacer 40 having a suitable width is applied or patterned in the emitter window 38. The inner spacer 40 thus has the task to define, on the one hand, the emitter region of the bipolar transistor and the region of the emitter terminal and to provide, on the other hand, an electrical insulation of the emitter terminal region from the base terminal region.
The base terminal, i.e. the electrical connection between the implanted “intrinsic” base region 36 and the base terminal layer 32 of a polysilicon material which, in general, is highly doped, is obtained by out-diffusing dopant atoms from the highly doped polysilicon material of the base terminal layer 32 into the silicon substrate 30. This doped region obtained by an out-diffusion of dopant atoms from the polysilicon material of the base terminal layer 32 into the silicon substrate 30 will subsequently be referred to as the “extrinsic” base terminal region 42.
Before the lateral out-diffusion of the extrinsic base terminal region 42 in the direction of the intrinsic base region 36, due to the required substrate etching, first of all diffusion in the depth of the substrate material 30 must take place to produce an electrical connection via the extrinsic base terminal region 42 between the base terminal layer 32 and the intrinsic base region 36 of the bipolar transistor.
In the manufacturing method known in the prior art, an in-diffusion of the doping atoms of at least 150 nm in depth, i.e. into the silicon substrate 30, is required since in the process of diffusion, the substrate etching of about 40 nm and the spacer length of about 100–150 nm, including the etching rate variations in the etching depth must be overcome to obtain the required diffusion depth. The base anneal process thus has to be performed at a relatively high temperature of about 1050° C.–1100° C.
Thus, a relatively high temperature budget (1050° C.–1100° C.) and thus a relatively high exposure to temperature for the semiconductor structure are required for this out-diffusion process, wherein additionally the terminal, i.e. the electrical connection, of the extrinsic base terminal region 42 to the intrinsic base region 36 can only be controlled relatively poorly. This leads to a relatively poor reproducibility of the electrical features of the semiconductor substrate, i.e. in particular of the base structure, of a conventional polysilicon bipolar transistor. Spurious scatterings of the values of the base terminal resistance of conventional polysilicon bipolar transistors can, for example, be the result.
As has been described above, the emitter window 38 of a polysilicon bipolar transistor having an implanted base is, for example, etched by means of fixed-time etching through the polysilicon material of the base terminal layer 32, wherein additionally etching into the single-crystal silicon substrate 30 takes place by means of suitably selected over-etching. Another possibility to pattern the emitter window 38 of a polysilicon bipolar transistor having an implanted base is to provide an endpoint detection of the etching process and suitably selected over-etching into the single-crystal silicon substrate 30. In both cases mentioned above, it is required for compensating layer thickness variations and etching rate differences over the silicon wafer to clearly etch the silicon substrate 30 considerably. This etching can, however, result in roughnesses in the substrate material, which as a result can, in an extreme case, cause an inhomogeneous interface in the emitter or an inhomogeneous base width and thus an inhomogeneous current distribution in the bipolar transistor.
Thus, important characteristic numbers, such as, e.g., the product of cutoff frequency fT and breakdown voltage UCE0 or the product of current amplification and early voltage, may deteriorate.
These problems occurring in the prior art when manufacturing polysilicon bipolar transistors having implanted bases, up to now, must be minimized by means of very complicated control measures when etching the emitter window.
In addition, it has been required when manufacturing polysilicon bipolar transistors to in-diffuse a relatively deep extrinsic base terminal region into the single-crystal silicon substrate due to the required etching of the silicon substrate and the resulting lower lying base region in the silicon substrate starting from the highly doped polysilicon material of the base terminal layer, wherein this is, for example, performed by a temperature process. In order to be able to out-diffuse the dopant, such as, e.g., boron, phosphorus or arsenic etc., sufficiently deep from the polysilicon material of the base terminal layer into the single-crystal silicon substrate, a relatively high temperature budget is required and thus a relatively high exposure to temperature of the semiconductor element cannot be avoided when manufacturing it. This is, however, very problematic, in particular in modern BiCMOS technology.
By etching the silicon material of the substrate region required in the prior art and due to the problems connected thereto when electrically connecting the polysilicon base terminal layer to the intrinsic base region, the result can be spurious scatterings of the values of the base terminal resistance of conventional polysilicon bipolar transistors. In addition, the electrical characteristics of bipolar transistors mentioned above, such as, e.g., the cutoff frequency, breakdown voltage, current amplification and early voltage thereof, are influenced negatively by the resulting roughness of the etched silicon substrate.